1. Field of the Invention
The present invention relates to optical lithography and, more particularly, a method for patterning subwavelength circuit features in a resist coated semiconductor wafer using phase-shifting mask techniques.
2. Description of the Related Art
As consumers demand more compact and more powerful electronic devices, manufacturers must fabricate ever smaller and denser integrated circuits. Heretofore, manufacturers have employed photolithographic techniques, i.e. techniques that utilize light (e.g. ultraviolet rays or monochromatic light), to pattern miniature circuit features such as, for example, gate electrodes, contacts, vias, metal interconnects, etc. in a substrate.
Photolithography involves the projection of a patterned image onto a coating or layer of photoresist (i.e., a radiation sensitive material) on a semiconductor wafer using an imaging tool and a photomask having a desired pattern thereon. After exposure, the resist-coated wafer is soaked in a developing solution so as to reproduce the imaged pattern. Depending on the particular type of photoresist (e.g., a positive or negative photoresist), a positive or a negative image of the pattern of the photomask is developed in the photoresist layer. For example, if a negative-tone resist is used, then the projected radiation passing through the photomask will cause the exposed areas of the photoresist to undergo polymerization and cross-linking. Upon subsequent development, unexposed portions of the photoresist will wash off with the developer, leaving a pattern of resist material constituting a reverse or negative image of the mask pattern. On the other hand, if a positive-tone resist is used, the radiation passing through the mask will cause the exposed portions of the resist layer to become soluble in a developer, thereby leaving a pattern that corresponds directly or positively to the transparent portions of the mask pattern. In either case, the remaining resist material will undergo subsequent processing steps such as, for example, etching and deposition to form the desired semiconductor devices.
To produce subwavelength circuit features, e.g., features smaller than the wavelength of the exposure radiation, manufacturers employ a photolithographic technique known as the phase-shifting mask technique. The phase-shifting mask technique uses a mask having a region that allows transmission of light therethrough and an adjacent region that shifts the phase of the light or radiation travelling therethrough by about 180 degrees relative to that of the incident light. In theory, this 180 degree phase difference causes destructive interference of light from the two regions along their interface to thereby enhance contrast of the projected image.
There are many variations to the phase-shifting mask technique, one common variation uses an attenuated phase-shifting mask (APSM). The APSM is typically made of a quartz substrate having a transmnissive region for transmitting light therethrough and an attenuating, phase-shifting region (i.e., an absorber) that absorbs or attenuates a portion of the incident light while phase-shifting and transmitting a remaining portion of the light therethrough. The absorber is typically formed of a transmittance controlling layer and a phase-shifting layer; optionally, the absorber may be formed of a single layer that performs both of these functions. The absorber may be made of molybdenum silicide oxynitride (MoSiON), chromium oxynitride (CrON), or chromium fluoride (CrF) with such thickness that allows about 5-15% light transmittance. In use, a mask or reticle (having a magnification such as, for example, 4.times.) is placed in a "stepper" that automatically and incrementally moves and exposes different portions of the wafer to form patterned images on the wafer. The patterned images in the photoresist are then developed into patterns by immersing the photoresist in a developing solution.
A major drawback of prior art APSM techniques is the destructive interference between the phase-shifted light and the non-phase-shifted light along the edge of the patterned image (in the image plane). This results in an intensity profile (i.e. intensity as a function of position in the image plane) that has "side lobes" (i.e., secondary lobe peak intensities) along the edge of the patterned image. In other words, the phase-shifted and the non-phase-shifted light interact along the edge of the patterned image such that there are regions of light with intensities that are significantly different from the background. An image with an intensity profile having sufficiently large side lobes could be developed into a pattern that has "ring" structures around the pattern features. The presence of such ring structures can degrade the intended shapes of the features and would thus prevent the printing of denser circuitry.
There is thus a need for an improved APSM process that significantly reduces or eliminates side lobes in the intensity profile along the edge of a patterned image.